METAMORPHIC ROCKS. 255 



beds of quartz of the Col de la Poissoniere, to the east of 

 Briancon ? ( 286 ) In the diamond districts of Minas Geraes 

 and St. Paul in Brazil, which have been recently studied 

 with great care by Clausen, the plutonic aetion of dioritic 

 veins has produced ordinary mica and specular iron in 

 quartzose itacolumite. The diamonds of Grammagoa are 

 contained in silicious beds, and are sometimes enveloped 

 in laminae of mica, like the garnets found in mica slate. 

 The most northern diamonds yet known, which have been 

 discovered since 1829, in 58 lat., on the European decli- 

 vity of the Oural, are geologically related to the black car- 

 boniferous dolomite of Adolphskoi, ( 287 ) and to augitie 

 porphyry ; but these relations have not yet been sufficiently 

 elucidated by exact observation. 



Among the most remarkable phsenomena of contact, I 

 may also allude to the formation of garnets in argillaceous 

 schist in contact with basalt and dolerite in Northumber- 

 land and the Island of Anglesea, and the production of a 

 large quantity and variety of beautiful crystals of garnet, vesu- 

 vian, augite and ceylanite, at the surface of contact of erupted 

 and sedimentary rocks ; namely, at the junction of the 

 syenite of Monzon with dolomite and compact limestone. ( 288 ) 

 In the Island of Elba, masses of serpentine, which perhaps 

 nowhere present such clear evidences of their eruptive cha- 

 racter, have produced sublimations of specular iron, and of 

 red oxide of iron, in fissures of calcareous sandstone. ( 289 ) 

 We still see specular iron formed daily from sublimation on 

 the sides of open fissures in the craters, and in recent cur- 

 rents of lava, of the volcanoes of Stromboli, Vesuvius, and 

 Etna. ( 29 ) The veins thus formed under our eyes by volcanic 

 forces, in rocks which have attained a certain degree of solidity, 



VOL. I. T 



